Egg white is a perishable product, even when stored under refrigeration, due to the growth of spoilage organisms. Most commercial pasteurization processes are intended to decrease the risks posed by pathogenic organisms such as Salmonella bacteria.
The temperatures necessary to achieve eradication of spoilage organisms are not easily, reliably reached without causing protein denaturation. Severe denaturation results in coagulation of the entire product, and this is considered undesirable because consumers prefer liquid, pourable products for convenience in a wide variety of recipes. Accordingly, the most typical commercial products are marketed in the frozen state and permit only one to two weeks stability at refrigerator temperature (4.degree. C). Until recently, refrigerator or room-temperature stable liquid egg products were not available. The present invention has special applicability to products based principally on egg white and provides an improvement over current, state-of-the-art technology.
Egg white, also referred to in the art as egg albumen, is actually a complex mixture of several different types of soluble protein. Significant among these are conalbumin, which is the second most abundant, and ovalbumin, the most abundant. All of the proteins will coagulate and lose their water solubility after heating for well-defined time-temperature combinations. However, not all proteins respond the same, and their responses can be altered by the presence of certain natural and added materials. For example, it is disclosed in European Patent Application 344,123 by Maley et al that whole eggs can withstand temperatures about 20.degree. F. higher than egg white. The onset of coagulation of egg white becomes a problem around 140.degree. F., due principally to the denaturation of the conalbumin.
In one early advance in egg pasteurization, Lineweaver and Cunningham disclosed in U.S. Pat. No. 3,251,697, that the addition of certain polyvalent metal salts enabled increasing the pasteurization temperature several degrees while not adversely affecting the physical properties of the egg. Similarly, in Food Products Formulary, Vol. 2, p.375, by Tressler and Sultan, it is indicated that salts of aluminum and iron can suppress coagulation by forming heat-stable complexes with conalbumin. However, this technique alone does not enable high enough heat treatments to achieve the stability necessary for reasonable periods of refrigerator storage.
With the addition of metal salts to formulations based essentially on egg white in U.S. Pat. Nos. 3,840,683 and 3,911,144, Strong and Redfern pasteurized at about 136.degree. F. for five minutes. These patents disclosed substantially cholesterol and egg yolk-free frozen egg products which had good freeze-thaw stability. This technology permitted the marketing and wide availability of a health-oriented product highly desired by many egg lovers. When frozen, the products last for extended times. It would be desirable, however, to improve the stability of such products against spoilage when maintained in a refrigerated condition.
Other early disclosures, primarily for frozen and dry egg products, employ hydrogen peroxide to aid in pasteurization. In both U.S. Pat. No. 2,776,214 to Lloyd et al and U.S. Pat. No. 3,364,037 to Mink et al hydrogen peroxide is added to egg prior to heating. The first of these discloses destroying natural catalase by heating egg white prior to adding the peroxide. The second adds an alkali. A later patent to Kohl et al (U.S. Pat. No. 3,615,705) combines these two teachings.
More recently, efforts have been made to produce products which remain stable for extended periods of refrigerated or room-temperature storage. For example, in U.S. Pat. No. 3,928,632, Glaser and Ingerson disclosed an aseptically-packaged, low-cholesterol egg product having an additive emulsion which is separately sterilized and homogenized prior to mixing with an egg component. No details of egg pasteurization are provided, but a lactylate salt is an essential ingredient.
In U.S. Pat. No. 4,971,827, Huang discloses that temperatures high enough to obtain a refrigerator-temperature-stable liquid egg product, can be employed when turbulent flow is achieved during pasteurization. To prepare the liquid egg product for heating, it is first heated to about 120.degree. F. and then homogenized. This is followed by a 2-stage heat process employing turbulent flow and resulting in a product temperature of 162.degree. F. The product is cooled and directly packaged without further homogenization. The example states that minimum denaturation was indicated by the percentage of water-soluble protein in the product.
In U.S. Pat. No. 4,853,238, Huang discloses a process which does not require turbulent flow even though even higher temperatures are employed. According to this latter process, microwave energy is used to heat a liquid egg composition in small diameter polytetraflouroethylene tubes at 185.degree. F. for 0.02 seconds to achieve pasteurization without undue coagulation or fouling of heat exchange surfaces. The heated liquid egg composition is rapidly chilled directly following heating. Again, a minimum functional loss in the finished product is reported and no homogenization following the pasteurization is disclosed. Because there is such a small hold time at the temperature identified as necessary and because microwave heating is often difficult to apply uniformly, rigorous quality control checks will be required to assure proper processing.
Swartzel et al, in U.S. Pat. Nos. 4,808,425, 4,957,759, and 4,994,291, disclose the preparation of shelf-stable whole egg products by high-temperature, short-time ultrapasteurization of liquid whole egg. Consistent with published procedures, they suggest heating under turbulent flow conditions. To improve pasteurization, they disclose the desirability of reducing the protein and fat unit size prior to heating. This is said to reduce any tendency of the product to coagulate. Denaturation is preferably kept as low as possible, but it is suggested that a homogenization step after heating be included for this whole egg product.